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Abstract:

A method and apparatus are used to simultaneously slice a multiplicity of
slices from a workpiece. The workpiece is held with a feed device so as
to position an axis of the workpiece parallel to axes of wire guide rolls
of a wire saw and is moved from above through a web of the wire saw. A
slurry is supplied as abrasive to wire sections of the web while the wire
sections are moved relative to the workpiece. The relative movement
guides the wire sections from an entry side to an exit side through the
workpiece. A coolant is sprayed from the side and below through nozzles
into slicing gaps in the workpiece. The nozzles are arranged below the
web parallel to the axes of the wire guide rolls. The coolant is sprayed
into the slicing gaps through a nozzle situated opposite the entry side
of the respective wire section.

Claims:

1. An apparatus for simultaneously slicing a multiplicity of slices from
a workpiece, the apparatus comprising: at least two cylindrical wire
guide rolls each having an axis and being disposed horizontally and
parallel with respect to one another and mounted rotatably about their
respective axes, each wire guide roll having grooves; a feed device; wire
that is guided in the grooves around the wire guide rolls so as to form a
horizontal web composed of a multiplicity of wire sections running
parallel to one another and in one plane between the wire guide rolls;
first nozzles disposed above the wire guide rolls and configured to spray
an abrasive onto the wire sections; and second nozzles disposed below the
web parallel to the axes of the wire guide rolls and configured to spray
a coolant into slicing gaps of the workpiece from the side and from
below.

2. A method for simultaneously slicing a multiplicity of slices from a
workpiece, the method comprising holding the workpiece with a feed device
so as to position an axis of the workpiece parallel to axes of wire guide
rolls of a wire saw; moving the workpiece, using the feed device,
perpendicularly from above through a web of the wire saw, the web being
formed from a multiplicity of wire sections running parallel to one
another and in one plane; supplying a slurry of hard substances in a
carrier liquid as abrasive to the wire sections while the wire sections
undergo a relative movement with respect to the workpiece as a result of
rotation of the wire guide rolls with a continual change in the direction
of rotation, which relative movement guides the wire sections from an
entry side to an exit side through the workpiece; spraying a coolant from
the side and from below into slicing gaps that arise during the movement
of the workpiece through the web, wherein the coolant is sprayed through
nozzles into the slicing gaps, the nozzles being arranged below the web
and parallel to the axes of the wire guide rolls, and wherein the coolant
is sprayed into the slicing gaps only through a nozzle situated opposite
the entry side of the respective wire section.

3. The method as recited in claim 2, wherein a further slurry of hard
substances in a carrier liquid is used as the coolant.

4. The method as recited in claim 3, wherein the abrasive used and the
coolant used have identical properties with the exception of the
temperature.

5. The method as recited in claim 2, wherein the abrasive used and the
coolant used have the same temperature.

6. The method as recited in claim 2, wherein the abrasive used and the
coolant used have different temperatures.

7. The method as recited in claim 2, wherein the temperature of the
coolant is altered during the movement of the workpiece through the web.

8. The method as recited in claim 7, wherein the temperature of the
coolant is altered in a manner dependent on an engagement length of the
wire sections in the workpiece.

9. The method as recited in claim 8, wherein the temperature of the
coolant is increased as the wire engagement length increases, and the
temperature of the coolant is reduced as the wire engagement length
decreases.

10. The method as recited in claim 2, wherein a volumetric flow rate of
the sprayed coolant is kept constant during the movement of the wire
sections through the web.

11. The method as recited in claim 2, wherein a volumetric flow rate of
the sprayed coolant is altered during the movement of the wire sections
through the web.

12. The method as recited in claim 11, wherein the volumetric flow rate
of the coolant sprayed in is altered in a manner dependent on the wire
engagement length in the workpiece.

13. The method as recited in claim 12, wherein the volumetric flow rate
of the coolant sprayed in is increased as the wire engagement length
increases, and is reduced as the wire engagement length decreases.

14. The method as recited in claim 2, wherein the workpiece is a single
crystal having a diameter of at least 450 mm.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to German Patent Application No.
DE 10 2012 201 938.3, filed Feb. 9, 2012, which is hereby incorporated by
reference herein in its entirety.

FIELD

[0002] The present invention relates to an apparatus and a method for
slicing a multiplicity of slices from a workpiece, in particular of
semiconductor wafers from a crystal, by means of a wire slicing lapping
method with alternating wire running direction.

BACKGROUND

[0003] Semiconductor wafers are slices composed of semiconductor materials
such as, for example, elemental semiconductors (silicon, germanium),
compound semiconductors (for example composed of an element of the third
main group of the periodic system such as aluminum, gallium or indium and
an element of the fifth main group of the periodic system such as
nitrogen, phosphorus or arsenic) or compounds thereof (for example
Si1-xGex, 0<x<1). They are required, in particular, as
basic material for electronic components and have to meet stringent
requirements with regard to flatness, cleanness and lack of defects.

[0004] Flat slices composed of other materials are required for other
applications, for example glass slices as substrates for producing
magnetic memory disks or slices composed of sapphire as a support for
manufacturing optoelectronic components.

[0005] Such slices composed of semiconductor material or some other
material are sliced from a rod consisting of the respective material. In
particular, a chip-removing machining method such as slicing lapping is
appropriate for slicing the slices. Chip removal or chipping is
understood to mean, according to DIN 8580, mechanical machining methods
in which material is brought to the desired form by removing excess
material in the form of chips. In this case, the term chip denotes a
particle detached from the workpiece.

[0006] According to DIN 8589, lapping is chipping using loose grain
distributed in a liquid or paste (lapping slurry) as abrasive, which is
guided on a normally shape-bearing counterpiece (lapping tool) with the
cutting paths of the individual grains being as far as possible
non-directional. The material removal is effected by brittle-erosive
separation of the material cohesion via the formation of micro cracks at
the penetration location of the lapping grain and spalling of small
material particles. Lapping is based on a three-body interaction between
workpiece, lapping grain and lapping tool. Lapping is characterized by
the fact that the tool carrier (lapping disk, lapping wire) does not
contain hard substances which come into engagement with the material in
chipping fashion.

[0008] Diamond, silicon carbide and aluminum oxide, in particular silicon
carbide, are important as lapping substances when slicing semiconductor
wafers.

[0009] In the case of single slicing lapping, exactly one slice is sliced
from the workpiece per cut; in the case of multiple slicing lapping, a
multiplicity of slices are sliced simultaneously per cut. Multiple
slicing lapping can be carried out using a wire which is multiply
diverted via rolls, such that it comes multiply into engagement with the
workpiece. This is then referred to as single-wire multiple slicing
lapping. Alternatively, methods are known in which a multiplicity of
individual wires, which are fixedly braced in a frame like strings of a
harp, work through the workpiece. This is then correspondingly referred
to as multi-wire multiple slicing lapping. The present invention relates
generally to the slicing of a multiplicity of slices from arbitrarily
shaped workpieces composed of arbitrary materials which can be machined
in chipping fashion. The invention relates particularly to the slicing of
a multiplicity of slices from prismatically shaped workpieces having
rectangular, hexagonal or octagonal base surfaces or of cylinders
composed of glass, sapphire or semiconductor material. Single-wire
multiple slicing lapping is described more thoroughly below. This is also
referred to in shortened designation as slurry wire sawing (SWMS "slurry
multi-wire slicing").

[0010] An apparatus for single-wire multiple slicing lapping ("slurry wire
saw") comprises as essential apparatus features wire, at least two wire
guide rolls arranged horizontally and parallel with respect to one
another, a take-off and a take-up spool, an apparatus for pre-tensioning
a wire in the wire longitudinal direction, a feed apparatus, by means of
which the workpiece can be fed perpendicularly to the axes of the wire
guide rolls toward the plane spanned by the axes, and an apparatus for
applying an abrasive in the form of a slurry of loose hard substances in
a carrier liquid. The wire guide rolls are cylindrical and mounted
rotatably about their longitudinal axes. Their lateral surfaces have a
multiplicity of grooves running concentrically about the axis and largely
equidistantly with respect to one another.

[0011] In the case of slurry wire sawing, the wire is guided under tension
by means of the grooves spirally multiply via the wire guide rolls such
that individual wire sections become situated in parallel fashion and
form a web. By rotating the wire guide rolls in the same sense, the wire
is unwound from the take-off spool and wound onto the take-up spool. In
this case, the wire sections of the web respectively move parallel to one
another in the wire longitudinal direction. In order to simplify the
explanation, the workpiece is assumed hereinafter to be a cylindrical rod
composed of semiconductor material (semiconductor rod). Said
semiconductor rod is adhesively bonded at its lateral surface to an
axially running strip composed of a sacrificial material (sawing strip),
for example composed of glass or graphite, and is clamped by means of the
latter and with its workpiece axis parallel to the axes of the wire guide
rolls in the feed apparatus.

[0012] By slowly feeding the rod parallel to the perpendicular of the
sawing web toward the web, the workpiece comes into contact with the web
by that section of its lateral surface which is situated opposite the
sawing strip, and a force builds up in the wire transverse direction
between tool (web) and workpiece. As a result of the relative movement
between workpiece and web on account of the sawing wire moved through the
apparatus, material removal is effected under pressure and with addition
of the abrasive. By maintaining the wire transverse tension by means of
further continuous feeding of the rod, the wire web works through the
entire cross section of the workpiece, and a multiplicity of slices are
obtained simultaneously.

[0013] Single-wire multiple slicing lapping can be effected with a
direction of movement of the wire sections of the web that is constant
over the entire cut, or with reversal of the direction of movement. In
this case, cutting with continual reversal of the wire direction is of
particular importance since specific disadvantages for the achieved
flatness and front/rear side parallelism of the slices obtained are
avoided by the reversal of direction. Asymmetries between the entry side
of the wire sections and the exit side of the wire sections can be
averaged out by the reversal of direction and thus are partly compensated
for, and the wire consumption can be reduced by the reversal of
direction.

[0014] The reversal of direction of the wire run corresponding to the
pilgrim step method ("pilgrim step motion", "wire reciprocation")
comprises a first movement of the wire in a first wire longitudinal
direction by a first length and a second movement of the wire in a second
direction, which is exactly opposite to the first direction, by a second
length, wherein the second length is chosen to be less than the first
length. For each pilgrim step, overall a wire length corresponding to the
sum of both lengths thereby runs through the workpiece, while the wire
section which comes into cutting engagement with the workpiece in this
case moves further only by a magnitude corresponding to the difference
between the two lengths from the take-off toward the take-up spool. In
the pilgrim step method, therefore, the wire is utilized multiply in a
ratio of the sum to the difference of the two lengths.

[0015] After working through the entire cross section of the workpiece,
the wire web reaches the sawing strip adhesively bonded onto the
workpiece. The further feed is stopped and the now multiply severed
workpiece is withdrawn again from the sawing web by reversal of the feed
direction. The workpiece has now been separated into a multiplicity of
slices which adhere by part of their circumference to the half-severed
sawing strip equidistantly and parallel to one another and perpendicular
to the workpiece axis. By chemical, thermal or mechanical release of the
adhesive bond, the slices are separated and supplied to a further
application-dependent subsequent processing.

[0016] The slurry wire sawing and an apparatus suitable for slicing
semiconductor wafers are described for example in EP 0 798 091 A2.

[0017] The flatness of the sliced slices that can be achieved by wire
slicing lapping is impaired by a multiplicity of effects. These include
effects related to the kinematics of the wire, the supply and
distribution of the abrasive in the sawing gap, the wear of the wire and
the sawing grain. Thermal processes have a particularly great influence
on the cutting result. It is known from DE 101 22 628 that chipping work
and friction processes bring about a heat input into the workpiece that
leads to an axial relative movement between the workpiece and the wire
sections. In a cylindrical workpiece, the length with which the sawing
wire is in engagement with the workpiece changes with the cutting
progress. The heat input and thus the axial relative movement between the
workpiece and the wire sections consequently changes slowly
(quasi-statically) with time. When cutting into and when cutting out of
the workpiece, abrupt changes in the engagement lengths are present, and
the cutting rate that results given a constant wire transverse tension is
particularly high. Therefore, a particularly great axial relative
displacement between workpiece and web occurs during cutting in and out,
with the result that all slices of the sawing cut acquire a flatness
deviation curved out of the ideal sawing plane substantially in the same
sense and to the same extent. This flatness deviation, referred to as
sawing-in and/or -out undulation, is particularly harmful since it has a
long wavelength (several centimeters) and in this case impairs the
parallelism of the front and rear sides of the slices (thickness
homogeneity) only to a small extent. Since the semiconductor wafers
exhibit largely elastic behavior in the range of centimeters, (or
longer), the sawing-in and/or -out undulation cannot be removed, or can
be removed only inadequately, by the material removal accomplished by the
subsequent processing steps.

[0018] Such undulatory slices are unsuitable for demanding applications.
In the case of slicing lapping of large, and particularly in the case of
slicing lapping of very large, workpieces into slices, these undesirable
thermally governed defects are particularly pronounced. Workpieces having
a large diameter are those whose area-equal circle upon projection of a
cross section along the principal axis with the smallest moment of
inertia has a diameter (equivalent diameter) of greater than or equal to
300 mm; workpieces having a very large diameter are those having an
equivalent diameter of greater than or equal to 450 mm.

[0019] JP 10180750 describes a method in which the temperature of the
abrasive supplied to the sawing gap by spraying from above onto the
sawing web is adapted in a closed control loop of temperature regulation
of the abrasive and temperature measurement and temporally variable
heating is thus counteracted.

[0020] DE 101 22 628 B4 describes a method in which the entire
part--situated above the wire web--of the lateral surface of the rod is
flushed with coolant that is temperature-regulated in a manner dependent
on time and cutting progress, and the rod is thus temperature-regulated.

[0021] EP 0 798 091 A2 describes methods in which the volumetric flow rate
of the abrasive supplied, the viscosity and the feed rate of the rod to
the sawing web are altered in a manner dependent on the cutting progress.

[0022] Finally, U.S. Pat. No. 7,959,491 B2 describes a method in which the
temperature of the slicing lapping agent supplied is increased
continuously steadily, but in a manner dependent on the instantaneous
position of the web in the rod, over the entire cutting progress from
sawing in until sawing out and partial compensation of the thermal
effects is thus performed in a manner dependent on the web position in
the rod.

[0023] The slicing gaps hidden in the rod can be observed if the workpiece
is pellucid or transparent at least in a certain spectral range. Thermal
observations of the slicing zone on a rod composed of silicon which is
transparent in the infrared spectral range showed, by means of a thermal
imaging camera, that the heat input into the sawing gap and over the
length of the sawing gap is not effected uniformly. In particular, it was
observed that the temperature in the slicing gap increases with the
engagement length from the wire entry toward the wire exit. The hottest
point is reached shortly before wire exit; directly at the surface upon
wire exit, the temperature decreases again somewhat, probably via thermal
emission and air convection at that surface of the rod which is close to
this point. The heating along the wire engagement thus takes place in a
very complicated way.

[0024] In the slicing gap of a workpiece composed of silicon, a
temperature increase of more than 20° C. is observed over the wire
engagement length, and just approximately 5° C. in the mass of the
surrounding silicon volume that has not yet been cut. The heat gradient
over the sawing gap reverses on a short timescale (a few seconds) if the
direction of wire movement reverses during slicing lapping preferably
carried out in the pilgrim step method. These dynamic temperature
fluctuations are considerable, take place for a short period of time with
the frequency of the pilgrim step and far exceed the averaged workpiece
temperature that varies only slowly over the cutting progress.

[0025] The known methods only compensate for this slow quasi-static
temperature change. They are unsuitable for compensating for the rapid
and much higher temperature changes and the effects thereof, in
particular the resulting undulation of the sliced slices.

SUMMARY

[0026] In an embodiment, the present invention provides

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Exemplary embodiments of the present invention are described in
more detail below with reference to the drawings, in which:

[0028]FIG. 1 shows an apparatus for slicing slices from a rod according
to the principle of single-wire multiple slicing grinding or single-wire
multiple slicing lapping.

[0029]FIG. 2 shows the distribution of abrasive and the profile of the
temperature over the wire engagement length L in the case of a sawing
wire running from left to right, when an apparatus in accordance with
FIG. 1 is used.

[0030]FIG. 3 shows the distribution of abrasive and the profile of the
temperature over the wire engagement length L in the case of a sawing
wire running from right to left, when an apparatus in accordance with
FIG. 1 is used.

[0031]FIG. 4 shows the distribution of abrasive and the profile of the
temperature over the wire engagement length L in the case of a sawing
wire running from left to right, at the instant when the wire section web
cuts into the rod, when an apparatus in accordance with FIG. 1 is used.

[0032]FIG. 5 shows the distribution of abrasive and coolant and the
profile of the temperature over the wire engagement length L in the case
of the sawing wire running from left to right, when abrasive is sprayed
from the left onto the web and coolant is sprayed from the right into the
sawing gaps by means of a nozzle arranged below the web.

[0033]FIG. 6 shows the distribution of abrasive and coolant and the
profile of the temperature over the wire engagement length L in the case
of the sawing wire running from right to left, when abrasive is sprayed
from the right onto the web and coolant is sprayed from the left into the
sawing gaps by means of a nozzle arranged below the web.

DETAILED DESCRIPTION

[0034] An aspect of the present invention is to specify an apparatus and a
method for simultaneously slicing a multiplicity of slices from a
workpiece with which the rapid and high temperature fluctuations during
the slicing lapping of the workpiece are compensated for as much as
possible and deviations of the actual cut surface from that of an ideal
cutting plane are avoided as much as possible.

[0035] The abrasive (slurry) is depleted in the slicing gap over the
length of the engagement from wire entry to wire exit on account of
displacement, dripping away and consumption and is distributed
non-uniformly there. Its concentration and composition change over the
length of wire engagement on account of grain wear and grain fracture. In
particular, the decreasing quantity of abrasive from wire entry to wire
exit leads to a decreasing width of the slicing gap and thus to a
thickness of the sliced slices that increases in a wedge-shaped manner in
the wire running direction.

[0036] Another aspect of the invention is to specify an apparatus and a
method which counteract depletion of the abrasive in the sawing gap over
the wire engagement length, such that slices having a thickness that
increases in the wire running direction do not arise.

[0037] In an embodiment, the present invention provides an apparatus for
simultaneously slicing a multiplicity of slices from a workpiece,
comprising wire, at least two cylindrical wire guide rolls arranged
horizontally and parallel with respect to one another and mounted
rotatably about their respective axes and having grooves, and a feed
device, wherein the wire is guided in the grooves around the wire guide
rolls such that a horizontal web composed of a multiplicity of wire
sections running parallel to one another and in one plane is present
between the wire guide rolls, furthermore comprising first nozzles, which
are arranged above the wire guide rolls, for spraying an abrasive onto
the wire sections and second nozzles, which are arranged below the web
parallel to the axes of the wire guide rolls, for spraying a coolant into
slicing gaps of the workpiece from the side and from below.

[0038] In another embodiment, the present invention provides a method for
simultaneously slicing a multiplicity of slices from a workpiece,
comprising holding the workpiece with an axis of the workpiece parallel
to axes of wire guide rolls of a wire saw by means of a feed device of
the wire saw; moving the workpiece by means of the feed device
perpendicularly from above through the web of the wire saw, wherein the
web is formed from a multiplicity of wire sections running parallel to
one another and in one plane; supplying a slurry of hard substances in a
carrier liquid as abrasive to the wire sections while the wire sections
describe a relative movement with respect to the workpiece as a result of
rotation of the wire guide rolls with a continual change in the direction
of rotation, which relative movement guides the wire sections from an
entry side to an exit side through the workpiece; spraying a coolant from
the side and from below into slicing gaps that arise during the movement
of the workpiece through the web, wherein the coolant is sprayed through
nozzles into the slicing gaps, which are arranged below the web parallel
to the axes of the wire guide rolls, and wherein the coolant is sprayed
into the slicing gaps only through the nozzle situated opposite the entry
side of the wire sections.

[0039] A further slurry of hard substances in a carrier liquid can be used
as the coolant.

[0040] The abrasive used and the coolant used can have identical
properties with the exception of the temperature.

[0041] The abrasive used and the coolant used can have the same
temperature. The abrasive used and the coolant used can have different
temperatures.

[0042] The temperature of the coolant can be altered during the movement
of the workpiece through the web. The temperature of the coolant can be
altered in a manner dependent on the wire engagement length of the wire
sections in the workpiece. The temperature of the coolant can be
increased as the wire engagement length increases, and reduced as the
wire engagement length decreases.

[0043] The volumetric flow rate of the coolant sprayed in can be kept
constant during the movement of the wire sections through the web. The
volumetric flow rate of the coolant sprayed in can be altered during the
movement of the wire sections through the web. The volumetric flow rate
of the coolant sprayed in can be altered in a manner dependent on the
wire engagement length of the wire sections in the workpiece. The
volumetric flow rate of the coolant sprayed in can be increased as the
wire engagement length increases, and reduced as the wire engagement
length decreases.

[0044] The temperature and the volumetric flow rate of the coolant can
simultaneously be altered according to the abovementioned stipulations.

[0045] The method is preferably used for producing semiconductor wafers,
particularly preferably for producing semiconductor wafers composed of
silicon, which have a diameter that is not less than 300 mm, for example
for producing wafers composed of silicon having a diameter of 450 mm.

[0046]FIG. 1 shows the elements of an apparatus for single-wire multiple
slicing lapping, comprising sawing wire 1, which, wrapped multiply
spirally around a left 3 and a right wire guide roll 4, is guided by
grooves 2 such that the wire sections running on the top side of the wire
guide rolls ("upper wire strands") run parallel and form a regular web 11
having constant distances between adjacent wire sections. A workpiece 15
is adhesively bonded to a sawing strip 16 by means of an adhesive 17. The
sawing strip 16 represents a feed device, by means of which the workpiece
is moved perpendicularly to the web 11 along arrow 18 and is brought into
engagement therewith. Furthermore, the apparatus comprises left nozzle
combs 19 and right nozzle combs 20 for supplying abrasive (slurry) in the
form of a left elongate jet 22 and a right elongate jet 23 onto the left
wire guide roll 3 and the right wire guide roll 4 and thus onto the web
11.

[0047] The wire guide rolls are mounted rotatably about axes 5 and 6.
Their axes and the axis 14 of the workpiece 15--a cylindrical rod in the
example shown--are oriented parallel to one another and run through the
corners of an isosceles triangle, the base of which connects the axes of
the wire guide rolls. In order to initiate the slicing process, one wire
guide roll, for example the left wire guide roll 3, is driven to rotation
7 ("Master"). The other wire guide roll ("Slave"), in the example the
right wire guide roll 4, concomitantly rotates, in a manner pulled by
wire 1, in the same sense in the rotation direction 8. In the example
shown in FIG. 1, the wire 1 is supplied from the left in the arrow
direction 9, runs in multiple alternation over the upper web 11 and in
the opposite direction over a resulting lower web 12 multiply via the
wire guide rolls and then finally runs out from the apparatus toward the
right in the arrow direction 10. The workpiece 15 is moved
perpendicularly in the direction 18 through the web 11 by means of a feed
device, which is represented by the sawing strip 16 in the illustration.

[0048] As soon as the underside of the workpiece 15 comes into contact
with the web 11, a force builds up between web 11 and the workpiece 15 in
the feed direction (=wire transverse direction; wire transverse tension).
By means of the relative movement of the wire sections running in the
same sense in the web 11 with respect to the workpiece, the lapping agent
sprayed onto the wire web and entrained by the wire sections, and the
wire transverse tension, material removal from the workpiece (and the
sawing wire) is brought about, and sawing gaps 13 form in which the web
11 works through the workpiece 15 upon further feeding in the arrow
direction 18.

[0049] In the pilgrim step sawing method, the direction of the wire
longitudinal movement 9, 10 is reversed multiply during a complete cut
through the workpiece 15, wherein, in each individual one of these
pairs--called "pilgrim step"--of direction reversals, the wire is moved
by a longer length in one direction and a shorter length in the opposite
direction. As a result, overall in each complete pilgrim step a wire
length corresponding to the sum of both lengths runs through the slicing
gaps; however, that length of the engaged wire section by which the
entire wire supply has thus shifted after one completely performed
pilgrim step from the take-off toward the take-up spool corresponds only
to the difference between these two lengths.

[0050]FIG. 2 shows essential elements of the apparatus in accordance with
FIG. 1 in side view, to be precise at the instant at which the wire 1 or
the wire sections of the web 11 move from left to right in the arrow
direction 9. The jet 22 sprayed onto the web from the left nozzle combs
19 and residues of the jet 23 sprayed from the right nozzle combs 20 and
still adhering to the wire are for the most part stripped away by the
surface of the workpiece when the wire enters into the workpiece. A
wire-entry-side zone 26 forms, in which the abrasive accumulates, and
only a small quantity of abrasive still remaining on the wire is
introduced from the wire into the sawing gap in order to bring about
material removal there.

[0051] Over the length L of the sawing gap 13, from wire entry to wire
exit from the workpiece, as a result of displacement, dripping away and
grain fracture, as measured from the location of the wire entry, a
progressive depletion of the abrasive S occurs, as is illustrated
schematically by curve 24 in the diagram S=S(L) in FIG. 2. The depletion
of the abrasive, the total quantity of the chipping work carried out over
the wire engagement length in the gap on the workpiece and the total
quantity of shear work carried out over the wire engagement length during
the movement of the wire against the viscous abrasive lead to a
temperature that increases from wire entry to wire exit in the sawing gap
in the wire longitudinal direction. This is shown schematically by curve
25 in the diagram T=T(L) in FIG. 2.

[0052]FIG. 3 differs from FIG. 2 in particular in that the wire running
direction, represented by arrow 26, is reversed and runs from right to
left. Curve 27 schematically shows the distribution S=S(L) of abrasive
over the wire engagement length L and curve 28 shows the profile T=T(L)
of the temperature over the wire engagement length L, as measured from
the location of wire entrance into the workpiece to the location of wire
emergence from the workpiece.

[0053]FIG. 4 shows that the distribution S=S(L) of the abrasive and the
profile T=T(L) of the temperature also change with the length of wire
engagement. If the workpiece has the cylindrical form shown, for example,
the wire engagement length L changes in a manner dependent on the path
length of the feeding of the workpiece. FIG. 4 shows the instant of
sawing into the workpiece, at which the length of wire engagement L is
particularly short. Accordingly, the distribution 37 of the abrasive and
the profile 38 of the temperature over the short wire engagement length L
are comparatively low. At the instant of engagement (the beginning of
cut), however, the change in the quantity of abrasive and the change in
the temperature are considerable.

[0054]FIG. 5 shows the distribution S=S(L) of the abrasive and of the
coolant and the profile T=T(L) of the temperature over the wire
engagement length L when carrying out the method according to the
invention. The apparatus according to the invention comprises, in
addition to the apparatus shown in FIG. 1, a left nozzle 29 and a right
nozzle 32. These nozzles are arranged below the wire web 11. The distance
between the nozzles is dimensioned such that the workpiece 15 has enough
space between the nozzles when working through the wire web.

[0055] The nozzles 29 and 32 respectively form an elongate, for example
cylindrical, comb whose axis 41 and 42, respectively, is parallel to the
axis 14 of the workpiece 15. The comb consists of a multiplicity of
punctiform individual nozzles or is embodied as an elongate nozzle slot.
The length of the nozzles 29 and 32 and the axial arrangement thereof
relative to the web 11 are such that the web is sprayed over its entire
width.

[0056] In the example of the method according to the invention as shown in
FIG. 5, the wire 1 and thus all the wire sections in the web 11 move from
the left, the abrasive supply being stripped away upon wire entry into
the workpiece in the zone 26 and being accumulated. The abrasive is
sprayed onto the web from above by the nozzle 19. In this phase, only the
nozzle 32 is switched on below the web 11. The nozzle 32 is that nozzle
below the web which is situated opposite the entry side of the wire
sections and sprays the coolant with a jet 30 from the side and from
below into the slicing gaps 13 in the workpiece 15.

[0057] Since the slicing gaps are narrow and become very deep with
increasing progression of cutting, the coolant sprayed in penetrates into
the slicing gaps only to a certain depth and thus only in a certain
region 31. As a result of the coolant being sprayed from the side and
from below into the slicing gaps, the region 31 acquires a particularly
large area and the cooling effect of the jet 30 of coolant becomes
particularly effective. The distance 43 between the nozzle 32 and the web
11, the distance 44 between the nozzle 32 and the surface of the
workpiece 15 and the width 45 and angular orientation 46 of the jet 30 of
coolant are preferably set such that a temperature-regulated region 31
arises in the slicing gaps, which leads to a profile T=T(L) of the
temperature over the wire engagement length L with the greatest possible
uniformity 40.

[0058] The coolant is preferably a slurry of cutting-active hard
substances in an aqueous or oil-containing or glycol-containing carrier
liquid. Particularly preferably, the coolant used is a slurry having the
same composition as the abrasive sprayed onto the wire web from above.

[0059] If such a slurry having the same composition is used, the spraying
of the coolant also has the effect that the side depleted of abrasive
receives additional abrasive. This then results in a distribution S=S(L)
of abrasive and coolant over the wire engagement length L which
corresponds to the curve 39 and is particularly uniform.

[0060] The left nozzle 29 and the right nozzle 32 are preferably arranged
such that their axes 41 and 42 and the axis 14 of the workpiece 15 form
the corners of an isosceles triangle, in which the axes 41 and 42 form
the base and wherein said base runs parallel to the wire sections of the
wire web 11.

[0061] In the example shown in FIG. 5, only the nozzle 19 on the entry
side of the sawing wire 1 into the workpiece sprays abrasive 22 onto the
web 11. The nozzle 20 on the opposite side remains switched off at this
point in time. Owing to the current wire movement in the arrow direction
9 from left to right, abrasive sprayed by the nozzle 20 would be
transported away from the workpiece by the movement of the wire sections
and would not manifest a slicing effect in the slicing gap. Alternate
spraying only from that nozzle 19 or 20 which respectively lies on the
side of the current wire entry into the workpiece is advantageous since
it saves abrasive.

[0062]FIG. 6 shows, complimentarily to FIG. 5, the situation if the wire
sections move from right to left in the arrow direction 26. Now only the
nozzle 29 situated opposite the entry side of the wire sections is
switched on below the web 11. Through said nozzle, coolant is sprayed
with a jet 33 into the slicing gaps 13 of the workpiece 15. Depending on
the distance between the nozzle 29 and the web 11, the distance between
the nozzle 29 and the surface of the workpiece 15, the width and the
angular orientation of the jet 33 and the volumetric flow rate of the jet
33 of coolant, a particularly effectively temperature-regulated region 34
arises in the slicing gaps 13. Therefore, also in the case of a direction
of the movement of the wire sections that is directed oppositely by
comparison with FIG. 5, this results in a temperature profile T=T(L) over
the wire engagement length L which corresponds to the curve 36 and is
particularly uniform. If a coolant is used whose composition corresponds
to that of the abrasive used, the distribution S=S(L) of abrasive and
coolant over the wire engagement length L, which corresponds to the curve
35, is likewise very uniform.

[0063] The method according to the invention preferably comprises
single-wire multiple slicing lapping carried out according to the pilgrim
step method, and a pilgrim step in the method according to the invention
preferably consists of a sequence of the steps illustrated in FIG. 5 and
FIG. 6:

[0064] In the first partial step, the sawing wire runs from left to right
in the arrow direction 9. In this case, a jet 22 of abrasive is sprayed
onto the web 11. At the same time, the right nozzle 32 arranged below the
web 11 and situated opposite the entry side of the wire sections is
switched on and sprays a jet 30 of coolant into the region 31 of the
slicing gaps 13. The nozzle 20 arranged above the wire web and the nozzle
29 arranged below the web are switched off in the meantime. As an
alternative to this, the nozzle 20 arranged above the wire web can also
be switched on.

[0065] In the second partial step, the sawing wire runs from right to
left. In this case, a jet 23 of abrasive is sprayed onto the web 11. At
the same time, the left nozzle 29 arranged below the web 11 and situated
opposite the entry side of the wire sections is switched on and sprays a
jet 33 of coolant into the region 34 of the slicing gaps 13. The nozzle
19 arranged above the wire web and the nozzle 32 arranged below the web
are switched off in the meantime. As an alternative to this, the nozzle
19 arranged above the wire web can also be switched on.

[0066] The optimum distances between the nozzles 29 and respectively 32
and the web 11, the optimum distances between the nozzles 29 and
respectively 32 and the surface of the workpiece, the optimum width and
angular orientation of the jet 30 and respectively 33 and the optimum
volumetric flow rate of coolant sprayed through the nozzles 29 and
respectively 32 are determined, for example, by test cuts with in each
case varied arrangements with assessment of the resulting temperature
distribution by means of a thermal imaging camera and the resultant
distribution of abrasive and coolant by means of measuring the wedge
configuration of the slices obtained in the wire longitudinal direction.
Such an optimization is to be performed individually for a type of saw,
since the number, size and distance of the wire guide rolls (web length),
the type and arrangement of the nozzles 19 and 20 and also the thermal
conditions of the surrounding machine housing are different depending on
the design series and, consequently, have a different effect on optimum
cooling and distribution of abrasive and coolant. The optimization can
also accompany production, that is to say be effected without any loss of
yield through test cuts.